In a production of acetic acid, when a solution containing water, hydrogen iodide, iodide ion (hereinafter, hydrogen iodide and/or iodide ion are/is sometimes simply referred to as hydrogen iodide), methyl iodide, methyl acetate, acetic acid, and others is distilled and purified, hydrogen iodide is condensed in a distillation column thereof due to interaction between hydrogen iodide and water. Accordingly, even when such a mixture is distilled, hydrogen iodide cannot be efficiently removed, and as a result, it is difficult to sufficiently reduce a concentration of hydrogen iodide in acetic acid as a final product. Moreover, high concentration of hydrogen iodide accelerates corrosion of the distillation column and peripheral equipment.
In order to reduce the concentration of the hydrogen iodide in the distillation column, it is proposed that a component such as methanol is supplied from a middle part or plate of the distillation column to convert hydrogen iodide to methyl iodide. Japanese Patent Application Laid-Open No. 40999/1994 (JP-6-40999A, Patent Document 1) discloses a process for the production of acetic acid which the process comprises:
feeding methanol and carbon monoxide to a carbonylation zone in which there is held a liquid reaction composition comprising: a rhodium catalyst; methyl iodide; an iodide salt; water at a concentration of up to about 10% by weight; methyl acetate at a concentration of at least 2% by weight; and acetic acid,
introducing the liquid reaction composition to a flash zone,
recycling the liquid fraction from the flash zone to the reaction zone, and recovering acetic acid product from the flash zone vapor fraction by use of a single distillation zone by:
introducing the vapor fraction from the flash zone into the distillation zone,
removing from the head of the distillation zone a light ends recycle stream, and
removing from the distillation zone an acid product stream having a water concentration of less than 1500 ppm and a propionic acid concentration of less than 500 ppm. This document also describes that the acetic acid product is passed through an ion exchange resin (anion exchange resin) bed to remove iodide contaminants. Incidentally, in Patent Document 1, the product acid stream is withdrawn from the bottom of the distillation zone or the second plate from the bottom of the distillation zone.
Further, this document mentions that the build up of hydrogen iodide component is prevented by introducing a small feed of methanol to the distillation zone, preferably below the feed point of the zone, to convert the hydrogen iodide to methyl iodide which is removed in the light ends recycle stream; up to 5000 ppm hydrogen iodide in the feed can be treated in this way; and by operating the distillation zone at a sufficiently elevated pressure, for the relatively high concentration of methyl acetate in the distillation zone, the hydrogen iodide is converted to methyl iodide which is removed in the light ends recycle stream.
However, even in these processes, hydrogen iodide cannot be effectively eliminated. Moreover, it is not preferred to increase pressure or temperature for converting hydrogen iodide to methyl iodide, because corrosion due to hydrogen iodide is accelerated. Incidentally, even when acetic acid having a water concentration of not more than 1500 ppm is removed, it is impossible to reduce the concentration of hydrogen iodide in the acetic acid to not more than 12 ppm due to affinity between hydrogen iodide and water (e.g., formation of the highest azeotrope). In addition, in order to further reduce the concentration of hydrogen iodide, it is necessary to use an anion exchange resin. As a result, the costs for the treatment increase. Moreover, since the product acid stream is removed at the base of the distillation zone, it is impossible to efficiently separate and recover the entrained rhodium catalyst. In addition, the operating trouble occurs due to blockage, or the quality of product is deteriorated, as well as it is unfavorable in costs. Therefore, it is difficult to conduct these processes industrially. Moreover, in order to avoid the above problems, equipment for separating or recovering the rhodium catalyst is required separately.
Japanese Patent Application Laid-Open No. 23016/1977 (JP-52-23016A, Patent Document 2) discloses a process for the removal and recovery of iodine-containing components and the drying of acetic acid, which comprises introducing a stream of acetic acid containing water, methyl iodide and hydrogen iodide into an intermediate point of a first distillation zone, removing, as an overhead fraction, from the first distillation zone most of the methyl iodide and portion of water, removing most of the hydrogen iodide from the bottom of the first distillation zone, withdrawing a stream from a middle section of the first distillation zone and introducing the stream into the upper section of a second distillation zone, introducing a stream of methanol into the lower section of the second distillation zone, removing from overhead of the second distillation zone a stream containing the remainder of water and methyl iodide present together with any methyl acetate, and withdrawing at or near the bottom of the second distillation zone a stream of a product acetic acid essentially dry and substantially free of hydrogen iodide and methyl iodide.
This document discloses that, in the process distilling the side cut fraction provided from the first distillation column in the second distillation column to recover acetic acid, the process does not require recycling of a side cut fraction, containing hydrogen iodide, from the second distillation column to the first distillation column since methanol is introduced into the second distillation column and hydrogen iodide is chemically removed.
Such a process does not require condensation of hydrogen iodide in the distillation column and removal of hydrogen iodide by side cut. However, the concentration of hydrogen iodide contained in product acetic acid cannot be sufficiently reduced.
Great Britain Patent No. 1350726 specification (Patent Document 3) discloses a purification process of a monocarboxylic acid component containing water and alkyl halide and/or hydrogen halide contaminants, which the process comprises introducing the monocarboxylic acid component containing water and alkyl halide and/or hydrogen halide contaminants into the upper half of a distillation zone, removing an overhead fraction containing most of water and the alkyl halide, withdrawing a stream from the middle part of the zone and below the point of the introduction of the zone to eliminate most of hydrogen halide present in the zone, and withdrawing a product monocarboxylic acid stream from the bottom part of the zone to obtain a product monocarboxylic acid stream being essentially dry and substantially free from any alkyl halide and hydrogen halide. The document describes that the liquid composition of carboxylic acid having a water content of about 3 to 8% by weight forms a peak of a hydrogen halide concentration at the intermediate part of the distillation column, and if a side stream is withdrawn from the intermediate part of the distillation column, a monocarboxylic acid in which almost all of hydrogen halide is removed can be obtained. Further, the document also discloses that in the case of subjecting a reaction product obtained from a reaction of methanol with carbon monoxide to a flash distillation, followed by introducing a fraction separated by the flash distillation into the distillation column, hydrogen iodide is condensed in a side stream from the intermediate part of the distillation column and removed.
However, in such a process, because of fluctuation of the peak position of hydrogen halide concentration due to variable factors (such as temperature, and pressure) in the distillation step, contamination of an acetic acid as a final product with hydrogen iodide sometimes occurs. Further, due to affinity between hydrogen iodide and water, it is impossible to remarkably reduce the hydrogen iodide concentration in the acetic acid.
Incidentally, WO02/062740 publication (Patent Document 4) discloses a continuous process for producing acetic acid comprising the following steps (a) to (d): (a) a step for reacting methanol with carbon monoxide; (b) a step for withdrawing a stream of a reaction medium from a reactor and vaporizing a portion of the withdrawn medium in a flashing step; (c) a step for distilling the flashed vapor utilizing two distillation columns to form a liquid acetic acid product stream; and (d) a step for removing iodides from the liquid acetic acid product stream such that the product stream has an iodide content of less than 10 ppb iodide by (i) contacting the liquid acetic acid product stream with an anionic ion exchange resin at a temperature of not lower than about 100° C. followed by contacting the resultant stream with a silver or mercury exchanged ion exchange substrate or (ii) contacting the liquid acetic acid product stream with a silver or a mercury exchanged ion exchange substrate at a temperature of not lower than about 50° C. As described above, according to the Patent Document 4, the iodides are removed from the acetic acid product stream with the use of the anionic ion exchange resin and/or a guard bed. However, in such a process, since it is impossible to remove carbonyl impurities having higher boiling points (such as an aldehyde, a carboxylic acid and an ester), a result in a potassium permanganate test which is a standard of a product acetic acid is deteriorated and the quality of the product is lowered. Further, it is difficult to remove higher boiling point components such as metal impurities and sulfate. Therefore, in order to obtain acetic acid satisfying a standard of a product acetic acid, it is necessary to install an incidental equipment for treating an acetic acid stream.
Incidentally, to remove iodides by a silver or mercury exchanged ion exchange resin is based on a monomolecular nucleophilic substitution (SN1) reaction, called as acetolysis, which is well-known in the field of organic chemistry. That is, a substrate iodide (RX) is decomposed into a carbonium ion (R+) and an anion (X−) by solvation of acetic acid (heterolysis), and the generated R+ quickly reacts with acetic acid as a nucleophilic reagent. This SN1 reaction is promoted by Ag, Hg or Cu which can form a coordinate bond together with an unshared electron pair of the leaving group (X).
In the above-mentioned SN1 reaction, the dissociation energy of the bond R—X becomes smaller as the carbon number of the R (or the carbon chain) increases (that is, in the order of CH3, C2H5, C3H7, C4H9 . . . ). For example, the dissociation energy of CH3I and that of C2H5I are 234 kJ/mol and 224 kJ/mol, respectively. Thus, higher iodide gets easily removable by a guard bed.
However, in the case where the R is hydrogen atom, it is known that the dissociation energy of the bond R—X is extremely large. For example, the dissociation energy of hydrogen iodide (HI) is 299 kJ/mol, and it is extremely difficult to separate and remove hydrogen iodide by a guard bed in principle. Moreover, thus obtained acetic acid is industrially and/or commercially insufficient in the light of the quality of the product and the corrosion of the equipment.
Japanese Patent No. 55695/1982 (JP-57-55695B, Patent Document 5) discloses a process for removing iodine from acetic acid, which comprises introducing an acetic acid stream containing iodine as an impurity into a first distillation column intermediate from the both ends thereof, introducing into the first distillation column intermediate from the both ends thereof a hydroxide of an alkali metal, and an acetate of an alkali metal, and/or hypophosphorous acid, withdrawing a product stream overhead from the first distillation column, introducing the product stream into a second distillation column intermediate from the both ends thereof, withdrawing an acetic acid stream substantially free from iodine from the lower part of the second distillation column, and withdrawing an overhead fraction containing iodine from the second distillation column. However, removal of hydrogen iodide by introducing the above reagent such as an alkali metal hydroxide into the first distillation column having a very high concentration of hydrogen iodide requires an extremely amount of the reagent. In addition, the amount of an alkali metal iodide generated by the treatment increases. Therefore, it is environmentally and economically disadvantageous in the light of separation or disposition. Moreover, since a large amount of potassium acetate generated from neutralization between acetic acid and the alkali metal hydroxide comes to be mixed in the product, the yield of acetic acid is deteriorated. Further, in the above-mentioned process, acetic acid is withdrawn as an overhead from the first distillation column. However, in the overhead, the proportions of hydrogen iodide and water are large. In such an environment, methyl iodide is hydrolyzed to generate hydrogen iodide, and the generated hydrogen iodide is allowed to be mixed in a feeding stream to the second distillation column.
[Patent Document 1] JP-6-40999A (Claims, and Paragraph Number [0043])
[Patent Document 2] JP-52-23016A (Claims, page 5, the lower right column, page 7, the lower left column to the lower right column, and Example 1)
[Patent Document 3] Great Britain Patent No. 1350726 specification (Claims, page 2, lines 66 to 76, and Example 1)
[Patent Document 4] WO02/062740 publication (Claim 1)
[Patent Document 5] JP-57-55695B (Claim 1)